Process for enhancing solubility and reaction rates in supercritical fluids

ABSTRACT

Processes for enhancing solubility and the reaction rates in supercritical fluids are provided. In preferred embodiments, such processes provide for the uniform and precise deposition of metal-containing films on semiconductor substrates as well as the uniform and precise removal of materials from such substrates. In one embodiment, the process includes, providing a supercritical fluid containing at least one reactant, the supercritical fluid being maintained at above its critical point, exposing at least a portion of the surface of the semiconductor substrate to the supercritical fluid, applying acoustic energy, and reacting the at least one reactant to cause a change in at least a portion of the surface of the semiconductor substrate.

RELATED PATENT DATA

This patent resulted from a divisional application of U.S. patentapplication Ser. No. 12/536,551, filed Aug. 6, 2009, entitled “Processfor Enhancing Solubility and Reaction Rates In Supercritical Fluids”,naming Theodore M. Taylor and Stephen J. Kramer as inventors, whichresulted from a divisional application of U.S. patent application Ser.No. 11/184,202, filed Jul. 19, 2005, entitled “Process for EnhancingSolubility and Reaction Rates In Supercritical Fluids”, naming TheodoreM. Taylor and Stephen J. Kramer as inventors, which is now U.S. Pat. No.7,598,181, the disclosures of which are incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to processes for enhancing solubility and reactionrates of in supercritical fluids, and more particularly to suchprocesses as they relate to the fabrication of semiconductor devices.

As the demand for ever-smaller silicon devices continues, and asresolution continues below the sub-micron level, the need for uniformand precise processes for depositing conductive pathways andinterconnects is increasing. It is also desirable for such processes toproceed rapidly. For example, it is frequently desired to formmetal-containing materials in and over semiconductor substrates. Themetal-containing materials can be incorporated into integrated circuitdevices, and/or can be utilized for formation of conductiveinterconnects between integrated circuit devices. There is also a needto provide substantially clean and defect free semiconductor substratesand surfaces onto which other materials may be deposited. Processes foruniformly and precisely etching and/or cleaning such substrates areneeded.

Wai et al, U.S. Pat. No. 6,653,236, describe methods of forming metalcontaining films over surfaces of semiconductor substrates using asupercritical fluid that contains metal forming precursors dispersedtherein. A supercritical fluid is a composition that exists in aquasi-liquid state above a defined critical pressure and a criticaltemperature for the composition. For example, carbon dioxide becomes asupercritical fluid at temperatures above 31° C. and pressures above1073 psi (73 atmospheres). Typical working conditions for supercriticalCO₂ are in the range of from about 60-100° C. and 1500-4500 psi. The useof supercritical fluids permits much greater amounts of precursorsand/or reactants to be dissolved or dispersed than prior CVD (chemicalvapor deposition) processes. However, reaction rates have been slowerthan predicted.

Others have used ultrasound in attempts to enhance reaction rates onsemiconductor substrates. For example, Hembree et al, U.S. Pat. No.6,224,713, describe a method and apparatus that uses ultrasonic waves towet etch silicon substrates to provide defect-free silicon structures.But, these methods have applied ultrasonic energy to traditionalreactions carried out in conventional liquids. It is well known that theapplication of ultrasonic energy to conventional liquids causescavitation and/or microbubble formation. It is these mechanisms thatenhance mixing of reactants to increase reaction rates. However,cavitation and microbubble formation are not possible usingsupercritical fluids. By definition, cavitation and gas bubble formationcannot occur in a fluid maintained at or above its critical point.

Accordingly, the need still exists to enhance reaction rates insupercritical fluids and to enhance reaction rates on or insemiconductor substrates. The need also exists for processes thatproduce uniform and precise results in the deposition or removal ofmaterials from the surfaces of semiconductor substrates.

SUMMARY OF THE INVENTION

The present invention meets these needs by providing processes forenhancing solubility and reaction rates in supercritical fluids, andmore particularly to such processes as they relate to the uniform andprecise deposition of metal-containing films on semiconductor substratesas well as the uniform and precise removal of materials from suchsubstrates. The processes may involve chemical reactions or may simplyenhance the solubility of materials on the surfaces of such substrates.

In accordance with one aspect of the present invention, a process forenhancing the rate of a reaction at the surface of a semiconductorsubstrate is provided and comprises, providing a supercritical fluidcontaining at least one reactant, the supercritical fluid beingmaintained at above its critical point, exposing at least a portion ofthe surface of the semiconductor substrate to the supercritical fluid,applying acoustic energy, and reacting the at least one reactant tocause a change in at least a portion of the surface of the semiconductorsubstrate. The acoustic energy may be applied directly to thesupercritical fluid, or may be applied to the semiconductor substrate(and thus indirectly to the supercritical fluid) through a suitableequipped wafer chucking or holding device. The applied acoustic energyis coupled through the semiconductor substrate to the supercriticalfluid. The reactant may comprise a chemical precursor such as anorganometallic compound. Alternatively, the reactant may comprise anetchant composition.

Preferably, the acoustic energy comprises ultrasonic energy. Theultrasonic energy may be supplied directly to the supercritical fluidfrom a probe located in or adjacent to the fluid or indirectly bycoupling an ultrasonic energy source to the semiconductor substrate.Preferably lower frequency ultrasonic energy is used and has a frequencyof between about 20 to about 100 kilohertz. The supercritical fluid maybe selected from the group consisting of carbon dioxide, ammonia, C₁through C₅ alcohols, C₂ through C₈ hydrocarbons, water, xenon, nitrousoxide, tetrafluoromethane, difluoromethane, tetrafluoroethane,pentafluoroethane, sulfur hexafluoride, CFC-12, HCFC-22, HCFC-123,HFC-116, HFC-134a, dimethylether, and mixtures thereof.

In accordance with one embodiment of the invention, the change in thesemiconductor substrate comprises the deposition of a layer of materialonto at least a portion of the surface of the semiconductor substrate.In this embodiment, the at least one reactant comprises ametal-containing composition such as, for example, an organometallicprecursor. The metal-containing composition may comprise one or more ofAl, Au, Ba, Co, Cr, Ga, Hf, In, Ir, Mo, Ni, Pt, Pd, Re, Rh, Ru, Sn, Sr,Ta, Ti, Tl, W, and Zr, including organic metal-containing precursors, orone or more semiconducting metals or materials such as B, Si, Ge, As,Sb, Te, Se, and their organic-containing precursors. Other agents suchas catalysts may also be present.

In accordance with another embodiment of the invention, the change inthe semiconductor substrate comprises etching at least a portion of thesurface of the semiconductor substrate. In this embodiment, the at leastone reactant comprises an etchant composition. The etchant compositionmay be selected from the group consisting of acidic etchants andalkaline etchants. Other agents such as surfactants and wetting agentsmay also be present.

In accordance with yet another embodiment of the invention, a method forenhancing the dissolution of a solute in a supercritical fluid isprovided and comprises providing a supercritical fluid maintained atabove its critical point, introducing a solute composition to thesupercritical fluid, and applying acoustic energy to the supercriticalfluid. In a preferred form, the process includes exposing thesupercritical fluid containing the solute to at least a portion of asemiconductor substrate, and reacting the solute to cause a change in atleast a portion of the surface of the semiconductor substrate. Theacoustic energy preferably comprises ultrasonic energy that may besupplied to the supercritical fluid from a probe located in or adjacentto the fluid or by coupling an ultrasonic energy source to thesemiconductor substrate. Preferably, the ultrasonic energy is has afrequency of between about 20 to about 100 kilohertz. As with previousembodiments, the supercritical fluid may be selected from the groupconsisting of carbon dioxide, ammonia, C₁ through C₅ alcohols, C₂through C₈ hydrocarbons, water, xenon, nitrous oxide,tetrafluoromethane, difluoromethane, tetrafluoroethane,pentafluoroethane, sulfur hexafluoride, CFC-12, HCFC-22, HCFC-123,HFC-116, HFC-134a, dimethylether, and mixtures thereof. For example, thesupercritical fluid may comprise carbon dioxide and the solutes maycomprise water and tetraethylorthosilicate.

Another embodiment of the invention provides a method for enhancing thesolubility of a solid on the surface of a semiconductor substrate. Thesolid may be in the form of a film, a residue, a coating, or particles.The method includes providing a supercritical fluid maintained at aboveits critical point in contact with the solid semiconductor substrate andexposing at least a portion of the semiconductor substrate having thesolid thereon to the supercritical fluid, applying acoustic energy,solubilizing at least a portion of the solid on the surface of thesemiconductor substrate, and removing the solubilized solid. The solidcan be from a number of different sources including photoresists, etchresidues, etch-produced polymers, metal contaminants, and sacrificialfilms.

Accordingly, it is a feature of the present invention to provideprocesses for enhancing the solubility of reactants and the reactionrates of solutes in supercritical fluids. It is a further feature of theinvention to provide processes that may be used to obtain uniform andprecise deposition of metal-containing films on semiconductor substratesas well as the uniform and precise removal of materials from suchsubstrates. These and other features and advantages of the inventionwill become apparent from the following detailed description andaccompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When working with liquids that are used in metal deposition or metalremoval processes, acoustic (ultrasonic) energy can be applied to theliquid to induce cavitation and microbubble formation, phenomena thatcan enhance reaction rates. However, when operating in fluids at abovetheir critical temperature and pressure points, there can be no gasformation, and thus there is no cavitation or microbubble formation.Further, when operating at the critical point of a fluid, there is nosound propagation. That is, there is total attenuation of appliedacoustic energy to the liquid.

We have found that one can operate above the critical point of a fluidwhere sound propagation again occurs. For example, when operating insupercritical CO₂ at 31° C., a graph of sound velocity (in meters/sec)versus pressure (in pounds per square inch) will show a drop in soundvelocity through the fluid as the pressure approaches 1073 psi (thecritical point) until the sound velocity approaches zero at the criticalpoint. However, as pressure increases above 1073 psi, sound velocitywill again increase. By operating at above the critical point in afluid, one can achieve sound propagation that is comparable to soundpropagation in conventional fluids.

We have determined that acoustic energy can be used to enhance bothsolubility and reaction rates in supercritical fluids. While not wishingto be bound by any particular theory, it is believed that pressureswings in the fluid are created by the leading and trailing edges of theacoustic waves as they propagate through the supercritical fluid medium.Such pressure swings overcome boundary layer effects at the surfacewhere reaction is desired. Specifically, the leading edge of apropagated wave creates a temporary increase in pressure which increasessolubility of the selected solute in the fluid medium. Likewise, thetrailing edge of the propagated wave creates a temporary pressure dropwhich results in the reduced solubility of a reaction product at asurface of interest. Such reduced solubility may result in theprecipitation of the reaction product. Thus, the application of acousticenergy to a supercritical fluid maintained at above its critical pointenhances bulk solubility of reactants in the fluid while also enhancingtransfer, including precipitation, of reactants out of solution at asurface of interest.

Because the attenuation coefficient for ultrasound increases withfrequency, it is preferred that any process be conducted at above thecritical point of the fluid, but using lower frequency ultrasonicenergy. A preferred frequency range for applied acoustic energy is fromabout 10 to about 10,000 KHz. Operation at higher frequencies ispossible, but attenuation becomes greater so there is less propagationof the acoustic energy through the fluid.

Depending on the desired process, acoustic energy may be provided in anumber of ways. For example, to enhance solubilization of reactants in abulk fluid, one or more conventional ultrasonic probes may be positionedin or adjacent to a sono-reactor to couple them to the fluid. Where itis desired to enhance a reaction at a surface of a substrate, the probeor probes may be positioned adjacent the surface. Generally, the probeshould be positioned within a few centimeters (from about 1 to about 10cm) of where the desired effect is to occur, either bulk dissolution ordeposition. Alternatively, acoustic energy may be imparted to a surfaceby coupling an ultrasonic transducer to the back side of a substrate.For example, the transducer may be in the form of a piezoelectric plateor block which is coupled to a substrate to cause the substrate tovibrate. Reactions at the opposite surface of the substrate are enhancedin this manner.

In one embodiment, the process of the present invention may be used toenhance dissolution of chemical precursors for supercritical fluiddeposition (SCFD) of metals, oxides, and nitrides. SCFD is a techniquefor depositing thin films onto semiconductor substrates. The processinvolves dissolving a precursor, for example tetra-orthoethylsilicate(TEOS) and a reactant, for example water, into a supercritical solventfluid such as carbon dioxide. The supercritical fluid containing thedissolved precursor and reactant is fed to a reaction chamber containinga heated semiconductor substrate where the precursor and reactant reactto form a thin film of silicon oxide. By controlling the temperature andpressure at which the precursor and reactant are mixed, the precursorand reactant can be dissolved into the supercritical fluid at below thetemperature at which a reaction occurs. Then, when the supercriticalfluid with dissolved reactants encounters the heated substrate, thereaction occurs causing a thin film of silicon oxide to deposit.

By applying relatively low frequency (10 to 10,000 KHz) acoustic energyto the supercritical fluid, the solubility of water is enhanced.Solubility in supercritical fluids is generally a strong function offluid pressure. The application of acoustic energy locally modulates thepressure in the fluid (which acts as a solvent) to provide increasedsolubility. The high mass transfer rate found in supercritical fluidsalso allows the dissolved precursor and reactant to rapidly diffuse awayfrom a source and into the bulk fluid. The acoustic energy also acts toovercome boundary layer effects at the surfaces of precursor andreactant sources to enhance the dissolution rate.

The process of the present invention may be utilized to solubilizeprecursors and reactants for the deposition of metals, oxides, nitrides,carbides, silicides, semiconductors, and other compounds, for enhancingsurface cleaning operations, and for enhancing reactions that take placeeither in the bulk fluid or at a substrate surface. The process may alsobe used to enhance the solubilization of organometallics,metal-containing compounds, semiconductor metal-containing compoundssuch as, for example, TEOS, chelated metals, water, metal alkoxides,surfactants, monomers, and oligomers prior to reaction in thesupercritical fluid.

In a preferred embodiment, the process may be used to deposit metalfilms onto semiconductor substrates such as those films taught by Wai etal, U.S. Pat. No. 6,653,236, the entire disclosure of which isincorporated herein. The metal-containing films can comprise, forexample, one or more of Al, Au, Ba, Co, Cr, Cu, Ga, Hf, In, Ir, Mo, Ni,Re, Rh, Ru, Sn, Sr, Ta, Ti, Tl, W and Zr. Alternatively, or in addition,the films can comprise semiconducting metals or compounds that includeB, Si, Ge, As, Sb, Te, and Se.

In this embodiment, the process includes exposing a surface of thesemiconductor substrate to a supercritical fluid. In the supercriticalregion there is only one phase, and it possesses properties of both gasand liquid. Supercritical fluids differ from traditional liquids inseveral aspects. For example, the solvent power of a supercritical fluidwill typically increase with density at a given temperature. Theutilization of supercritical fluid can reduce a temperature at whichmetals are deposited relative to other methods, and yet can enhance adeposition rate of the metals. Additionally, deposition from within asupercritical fluid can allow for infiltration of very small, highaspect ratio features. Thus, the process can be used to fill sub-micronnano-features.

The supercritical fluid may comprise one or more of CO₂, ammonia, or analkanol having from one to five carbon atoms. Exemplary alkanols areethanol and methanol. Other exemplary materials that can be formed intosupercritical fluids are isooctane, hexane, heptane, butane, methane,ethane, propane, ethene, propene, water, xenon, nitrous oxide,tetrafluoromethane, difluoromethane, tetrafluoroethane,pentafluoroethane, sulfur hexafluoride, CFC-12, HCFC-22, HCFC-123,HFC-116, HFC-134a, and dimethylether.

For particular applications the supercritical fluid can comprise CO₂,and can include various components, including, for example, a catalystsuch as an H₂-activating catalyst dissolved therein. An advantage ofutilizing CO₂, as opposed to other supercritical fluids, is that CO₂ hasa relatively low critical temperature of 31° C. The metal-containingprecursor utilized in particular aspects of the present inventioncomprises a metal (such as, for example, Al, Au, Co, Cr, Cu, Hf, In, Ir,Mo, Ni, Rh, Ru, Sn, Ta, Ti, W and/or Zr) in combination with a chemicalgroup (which can be referred to as a ligand) which enhances solubilityof the metal in the supercritical fluid. Suitable ligands includeβ-diketones of the general formula R₁C(O)CH₂C(O)R₂, in which R₁ and R₂can be fluorinated or non-fluorinated alkyl groups. Exemplaryβ-diketones are acetylacetone, trifluoroacetylacetone,hexafluoroacetylacetone, thenoyltrifluoroacetone, andheptafluorobutanoylpivaroylmethane. Other suitable ligands include6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate andtris(2,2,6,6-tetramethyl-3,5-heptanedionate.

For applications in which the supercritical fluid comprises CO₂, thechemical group utilized to enhance solubility of a metal within thefluid can comprise a fluorocarbon, with an exemplary group beinghexafluoroacetylacetone. An exemplary copper-containing precursor iscopper(II) hexafluoroacetylacetonate (Cu(hfa)₂). Additionally, oralternatively, the metal-containing precursor can comprise one or moreof cobalt(II) hexafluoroacetylacetonate, nickel(II)hexafluoroacetylacetonate, and gold(II) hexafluoroacetylacetonate.

The H₂-activating catalyst is a material which can react with H₂ to format least one activated hydrogen species. The catalyst can include, forexample, noble metal catalysts, and can, in particular applications,include at least one of palladium, platinum, rhodium, iridium andruthenium. In particular applications, the catalyst can comprise one ormore of palladium, platinum, titanium; tin and ruthenium. For example, asingle catalyst can be provided, and such catalyst can be palladium.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

1. A method for enhancing the solubility of a solid on the surface of asemiconductor substrate comprising, providing a supercritical fluidmaintained at above its critical point in contact with said solidsemiconductor substrate, exposing at least a portion of saidsemiconductor substrate having said solid thereon to said supercriticalfluid, applying acoustic energy by coupling an energy source directly tosaid semiconductor substrate, solubilizing at least a portion of saidsolid on said surface of said semiconductor substrate, and removing saidsolubilized solid.
 2. A method as claimed in claim 1 in which said solidis selected from the group consisting of photoresists, etch residues,etch-produced polymers, metal contaminants, and sacrificial films.
 3. Amethod as claimed in claim 1 in which said acoustic energy comprisesultrasonic energy.
 4. A method as claimed in claim 3 in which saidultrasonic energy is supplied to said supercritical fluid from a probelocated in said fluid.
 5. A method as claimed in claim 3 in which saidultrasonic energy is supplied to said supercritical fluid by coupling anultrasonic energy source to said semiconductor substrate.
 6. A method asclaimed in claim 3 in which said ultrasonic energy is has a frequency ofbetween about 20to about 100kilohertz.
 7. A method as claimed in claim 1in which said supercritical fluid is selected from the group consistingof carbon dioxide, ammonia, C₁ through C₅ alcohols, C₂ through C₈hydrocarbons, water, xenon, nitrous oxide, tetrafluoromethane,difluoromethane, tetrafluoroethane, pentafluoroethane, sulfurhexafluoride, CFC-12 , HCFC-22 , HCFC-123 , HFC-116 , HFC-134a,dimethylether, and mixtures thereof.
 8. A method as claimed in claim 1in which said acoustic energy comprises ultrasonic energy which issupplied to said supercritical fluid by coupling an ultrasonic sourcedirectly to said semiconductor substrate.